This invention relates to fire retardant polymers and more specifically to such polymers having a low halogen content.
There is an immediate need for fire retardant thermoplastic resins, especially fire retardant styrene polymers having a relatively low halogen content and producing a low halogen smoke. This type of styrene polymer is required especially for use in fabricating furniture and television cabinets. Although many halogen-containing additives have been employed in thermoplastic polymers to yield some degree of fire retardance, most employ substantial amounts of halogen.
Two modes of fire retardance mechanisms are known, namely, halogen/antimony-type gas-phase flame inhibition and formation of a pyrolysis char that inhibits flame propagation. In the latter method certain bifunctional cross-linking agents such as 4,4'-bis(chloromethyl)diphenyl oxide have been used as additives to promote char. In the former process, the halide combines in the gas phase with the free radicals produced during combustion. Thus, free radicals are prevented from propagating the burning reaction and combustion stops.
As noted by Lyons, "Chemistry and Uses of Fire Retardance," Interscience Publishers, 1970, a strictly halogen mechanism typically requires at least 12 to 15% chlorine before flame retardance is observed. The same source reports that at least 7% chlorine is necessary in the presence of Sb.sub.2 O.sub.3 for flame retardance. In addition, the work of Fenimore, et al., "Combustion and Flame," Vol. 10, 1966, serves to place a lower limit on the halogen content necessary in the art recognized halogen/antimony mechanism. Fenimore et al. examined known ways of making polymers less flammable. It was found that a chlorinated polyethylene composition with greater than 7% chlorine did possess flame retardant characteristics in the presence of Sb.sub.2 O.sub.3. A composition containing 2.4% chlorine, however, was found to have too little chlorine to develop the inhibitory action of antimony fully. Thus, this mechanism would require a minimum of 7% halogen, and possess the attendant disadvantage of producing high halogen content smoke.
It is desirable, therefore, to develop polymers requiring less halogen to develop flame retardant characteristics. Indeed, a halogen independent mechanism is needed to completely eliminate the harmful effects of halogen containing smoke. I have found such a halogen independent mechanism. Even when vinylbenzylchloride is employed in my invention, the halogen levels are significantly lower than in the prior art.